The present invention relates to a control system for a dry-plate friction clutch for an automatic transmission of a motor vehicle.
A control system for automatically controlling a friction clutch is known, which is controlled to be engaged for starting the vehicle and to be released before stopping of the vehicle.
Japanese Patent Application Laid-Open No. 60-78119 discloses a clutch control system where a clutch is gradually engaged at the start of the vehicle by operating the clutch at three different speed stages. The operation proceeds from the first stage to the second stage, and from the second stage to the third stage when vehicle speed reaches respective predetermined reference speeds.
However, in the above-described system, it is difficult to detect the position of a clutch plate from which the second stage starts. Moreover, although the third stage starts at the predetermined vehicle speed, the clutch may not be sufficiently engaged at the stage. Consequently, the complete engagement of the clutch at the end of the third stage causes a shock. If such a control system is used in with a continuously variable transmission, shock occurs twice, that is, at the lock-up of the clutch and at a start of the upshifting of the transmission, which reduces the driveability of the vehicle.
In addition, since the speed stages are fixed regardless of the differences in engine load at the start of the vehicle, the partial engagement state becomes longer than necessary at low engine load, and the slipping period of the clutch increases at heavy engine load.